I had a close issue of the same magnitude.
Tab order worked in Preview PDF
Cause: I discovered that a HyperLink e-mail address that was put in while designing the form in MS WORD was the cause
Solution: Changed the hyperlink to regular text. I made it blue and underlined it (Looks like a hyperlink). Saved form as PDF
and opened it up back in Adobe Live Cycle. Customized tab order slightly by going to Window > Tab Order and making custom modifications.
Issue is resolved.
What if i want to restrict user from using the arrow keys within radio button??
I have 3 options in on radio group where the 2nd and 3rd options are protected. Ofcourse tab order does not work there but when the focus is on the first option, i am still able to move to the next options (2nd and 3rd) with arrow keys and they get selected despite them being "protected".
This happens only when i use arrow key and now while using mouse!